A Practical Dynamic Modulus Testing Protocol

The dynamic modulus test is widely accepted by pavement agencies as the critical parameter for the recently proposed mechanistic empirical design procedure and the candidate of the simple performance test to accompany the Superpave volumetric mix design process. However, the specified dynamic modulus test procedure is time-consuming and costly. State pavement agencies are seeking a more practical test protocol. This paper presents a method for identifying a practical dynamic modulus testing procedure. The currently well-adopted method of calculating the dynamic method is discussed and compared to the more fundamental dynamic modulus calculation method by using actual experimental data from two different asphalt mixtures. It was found that the NCHRP report proposed method produces higher modulus values, but the difference is less than 10 %, as indicates that the simple peak to peak method can be used in the calculation without compromising accuracy. A comprehensive dynamic modulus test, which incorporates strain level, test temperature, and frequency, was performed on one asphalt mixture. Experimental data were analyzed with t-test at the 95 % level of confidence. The analysis results show no statistical difference between the dynamic modulus for the two studied strain levels and no permanent damage was found on tested specimens for all three test temperatures. Comparison of the master curves built by different temperature and frequency combinations illustrates redundancy in test temperature and frequency. A more practical dynamic modulus test procedure is proposed based upon the evaluation. This research shows that three test temperatures, 4.4°C, 21.1°C, and 37.8°C, and six frequencies, 25, 10, 5, 1, 0.5 and 0.1 Hz, plus one additional frequency of 0.01 Hz at 37.8°C are adequate to build a smooth master curve to satisfactorily characterize asphalt mixtures.